The present invention relates to a rubberized asphalt composition to be used for the formation of a road pavement, and process for producing same.
Up to now so-called rubberized asphalts or additives for producing various kinds to make so-called rubberized asphalt composition have been developed and put on the market. However, almost all of them are used in a non-vulcanized state, that is, in the state, in which no cross-linking reaction occurs. But, the term "rubber" should originally be understood as a material of elastic properties, which are to be obtained by the cross-linking reaction due to the vulcanization. Therefore, rubberized asphalt composition or rubberized asphalt mixture obtained by mixing the rubberized asphalt compositions with aggregates could not be said to be "rubberized" in the true meaning of the word, since the cross-linking reaction did not occur in them. Heretofore there have been made, of course, many trials for obtaining vulcanizable rubberized asphalt composition, but in vain. The reason was found in that conditions for cross-linking due to the vulcanization, including the selection of curing agents, could not properly be settled for practical operations of the road pavement.
In the recent development in the art of rubberized asphalt compositions for producing rubberized asphalt mixtures to be used for road pavement, Japanese patent publication No. 22319/1968 is noticeable because the idea of producing cross-linking type rubberized asphalt compositions has been disclosed. In this prior art there is proposed a method of producing rubberized asphalt composition, in which asphalt is combined with urethane, which is to be obtained by the reaction of isocyanate with an intermediate diene polymer having allylic hydroxyl groups as functional groups at the terminals of the main chain, and it is characterized in that said urethane is at least partially uncured when combined with the said asphalt. Further, this prior art has advantages that the intermediate polyhydroxy polymer has a viscosity at 30.degree. C. of about 5-20,000 poises, preferably about 15 to 5000 poises and an average molecular weight of 400 to 25,000 and is liquid or semi-solid flowable at ambient temperatures. This prior art could be said to be epoch-making in that the rubber material proposed by this prior art is a liquid rubber of low molecular weight and the cross-linking reaction is effected by the combination of reactive allylic hydroxyl group contained at the terminals of the main chain with isocyanate as a curing agent, whereby paving mixtures having elastic properties should be obtained.
However, there have been not yet known any practical examples, in which cross-linking type rubberized asphalt mixtures have been produced according to the method of the said prior art and applied for practical road pavement operations. This was also due to the fact that the conditions of producing rubberized asphalt composition according to the method of the prior art does not match with the conditions required for practical paving operations.
As already mentioned, the said prior art is characterized in that, when combining asphalt with urethane obtained by the reaction of an intermediate diene polymer having allylic hydroxy groups with isocyanate, said urethane remains in an at least partially uncured state. On the other hand, there is described in the specification of the prior art that the ability of the intermediate diene polymer to react with isocyanate at normal temperature of asphalt pavement, that is, in the range of about 120.degree. F. (about 49.degree. C.)-200.degree. F. (about 94.degree. C.) appears to be largely affected to this more reactive allylic configuration. From the above matters it is easily understandable that the reaction of the intermediate diene polymer with isocyanate is effected by heating, but the reaction should proceed in such way that the urethane to be obtained by the reaction remains in a at least partially uncured state, till said urethane is combined with asphalt. In other words, the at least partially uncured state of the urethane, as is characterized by the prior art, can be maintained by the condition that the reaction is carried out at a low temperature of 120.degree. F. (about 49.degree. C.)-200.degree. F. (about 94.degree. C.).
However, this heating condition does not match with the practical conditions for producing asphalt mixtures to be used for the road pavement. According to the practice of an asphalt mixing plant, it is usual that asphalt is heated to a range of 150.degree. C.-165.degree. C. This temperature range should be regarded as a requisite in consideration of the temperature condition required for the paving operation of the asphalt composition.
According to our experiments it was made clear that the reaction of the intermediate diene polymer with isocyanate was finished at a temperature of 140.degree. C., and the urethane obtained thereby is completely cured. Therefore, the process of Japanese patent publication No. 22319/1968 could not be put on practice.